Fluid applicators are used in a variety of applications in which a fluid, such as an adhesive, is dispensed onto a substrate in discrete quantities. In many such applications, the fluid is dispensed according to specific requirements and precise tolerances with respect to quantity and area of coverage. Accordingly, it is desirable to dispense the fluid in consistent quantities throughout a discrete dispensing operation.
Some conventional fluid applicators have a valve stem with a valve body or ball on its distal end that is disposed on an upstream side of a valve seat. In operation, the valve ball is moved in an upstream direction to open the valve and in a downstream direction to close the valve. With this type of fluid applicator, when the valve opens, the valve ball is moving in an upstream direction against the direction of fluid flow and has a tendency to delay and disrupt the flow of fluid out of the dispensing nozzle. Similarly, when the valve closes, the valve ball is moving in the downstream direction with the direction of fluid flow and the upstream valve ball has a tendency to cause a small additional quantity of fluid to be dispensed.
In order to dispense fluid with sharply defined boundaries and with consistent quantity and area of coverage, it is necessary that the motion of the valve ball be very fast, and the fluid flow be abruptly started and stopped to obtain sharp, square, cut-on and cut-off edges. Thus, the tendency of the upstream valve ball to dispense even small quantities of fluid after the gun and valve have been shut off is undesirable.
To account for the aforementioned deficiencies of the upstream valve ball type of fluid applicator, “snuff-back” valves have been developed to buffer or capture any stringing material or other post shut-off fluid flow before it is undesirably deposited on the substrate. In one such snuff-back valve, a valve tip is disposed in a cavity on a downstream side of a valve seat. The valve tip is moved in the downstream direction away from the valve seat to open the valve and allow fluid to flow into the cavity. From the cavity, the fluid then passes through a fluid outlet in the cavity and thereby is dispensed onto a substrate. To close the valve, the valve tip is moved in the upstream direction toward the valve seat. Consequently, as the valve opens, the valve tip is moving in the same downstream direction as the viscous fluid and the fluid begins to be dispensed simultaneously with the opening of the valve tip. When the valve closes, the valve tip is moving in the upstream direction and is effective to sharply cut-off the flow of viscous fluid.
While such snuff-back valves operate effectively to provide sharper cut-on and cut-off of fluid flow, they may suffer from an undesirable effect known in the industry as “hammerhead.” Hammerhead occurs when the initial quantity of dispensed fluid is significantly and undesirably greater than the quantity of fluid dispensed during the remainder of the dispensing operation. For example, in a dispensing operation for a linear application of fluid, the hammerhead effect may cause an undesirable “blob” of fluid at the beginning of the line of fluid while the rest of the line comprises the desired quantity and coverage of fluid. In the above-described type of snuff-back valve, the hammerhead effect is caused by a pressure in the cavity resulting from the initial downward movement of the valve tip during the start of a dispensing operation. In other words, in the initial moments of a dispensing operation, as the cavity fills with fluid and the valve tip continues to move further downward into the cavity, the resultant pressure in the cavity causes a large initial quantity of fluid to be dispensed from the fluid outlet in the cavity (i.e., the aforementioned hammerhead effect).
These and other shortcomings are addressed in the present disclosure.